Reverse temperature field rolling method for mg alloy sheet

ABSTRACT

A rolling technique for manufacturing magnesium alloy sheets with high plasticity which is characterized in that by heating the rolls during rolling to ensure an unconventional decreasing temperature field from the surface to the center within the rolled material. Due to the poor plasticity and formability of magnesium and magnesium alloys, the use of their sheet products are currently limited, in particular, in high performance applications. To solve this problem, improving on the plasticity and formability of magnesium and magnesium alloy sheets is urgently needed. The present invention presents a rolling technique to manufacture magnesium and magnesium alloy sheets with high plasticity. The said rolling method is featured by heating the rolls during rolling to ensure an unconventional decreasing temperature field from the billet surface to its center, and to make the roll temperature to match with the initial temperature field of the billet material. By the said unconventional decreasing temperature field it is meant that the billet surface temperature is higher than the temperature in its center, and there is heat transfer from the surface to the center, during rolling.

THE TECHNICAL FIELD OF THE PRESENT INVENTION

The present invention relates to a rolling technique for manufacturing magnesium alloy sheets, which is featured by heating the rolls during rolling to ensure an unconventional decreasing temperature field from the surface to the center within the billet material.

BACKGROUND OF THE INVENTION

Magnesium and magnesium alloys are the lightest metallic materials with great potential for structural applications including aerospace, military arms, automotive, materials handling, communication, and portable electronic appliances. However, magnesium and magnesium alloys exhibit low ductility and poor formability for deformation processing, which has been the major obstacle to the development of the magnesium industry.

For structural applications, the cast products of most metallic materials usually take up only about 10-15% of market share, while the products manufactured by deformation processing can account for as high as 80% of market share. It is well known that magnesium and magnesium alloys are very difficult to shape by deformation processes, not only for primary deformation processing starting from as-cast ingot materials, but also for secondary deformation processing to form components.

The primary deformation processes include:

Extrusion for manufacturing bars, rods, tubes, shapes, and the like;

Drawing for manufacturing wires, tubes, and the likes;

Rolling for manufacturing plates, sheets, rods, and the like;

Forging for manufacturing solid components;

Among the above-mentioned primary deformation processes, the most difficult is the rolling of magnesium and magnesium alloy sheets (0.3-2.0 mm in thickness) directly from cast billets. With the use of the conventional rolling process, the material utilization ratio is only about 30-40%. Besides, it requires a relatively slower rolling speed and the deformation amount for each pass of rolling is quite limited. Therefore, the production cost is high. Since sheet products are the most commonly used materials for shaping components by secondary deformation processing, it appears that the rolling for manufacturing magnesium and magnesium alloy sheets is the key technology which affects the development of the magnesium industry.

Due to the limited plasticity and formability of the magnesium alloy sheets manufactured by conventional rolling techniques, re-heating is usually required while they are formed by the secondary deformation processes. This is not the case with low-carbon steel and aluminum sheets, which can be shaped into products by cold forming processes. Because of the high forming cost and poor surface quality associated with re-heating during forming, magnesium alloy sheet products have been rarely used.

Therefore, it is the common goal for scientists all over the world to improve the plasticity of magnesium alloy sheets, to increase the material utilization ratio, and to enhance the production efficiency for manufacturing magnesium sheet products. Unfortunately, no significant breakthrough with respect to this issue has been achieved so far.

Magnesium has a hexagonal crystal lattice structure, which gives rise to a poor plasticity. To improve the ductility of Mg alloys, alloying by the addition of such elements as Li, In, or Ag, has been tried, but little result has been achieved. On the other hand, it is known that enhancing the purity of the magnesium and magnesium alloys can result in ductility improvement, but this will increase the production cost significantly, and thus is not applicable for production practice on industrial scale. Therefore, grain refining is the most practical way to improve the ductility or plasticity of magnesium alloys.

It has been shown by recent studies that the plasticity of magnesium and magnesium alloys can not be improved significantly unless the grain size is refined down to less than 10 μm. Besides, the microstructure should be homogeneous and there are no abnormal coarse grains. That is to say, only those techniques which can achieve such a grain refinement effect and ensure a uniform, homogeneous microstructure can result in significant and steady improvement on the plasticity of magnesium alloys.

By using the conventional hot rolling or cold rolling techniques, it is difficult or hardly possible to manufacture magnesium alloy sheets with both a homogeneous microstructure and fine grains of less than 10 μm in average size.

Up to present, the major ways to refine grain size of magnesium alloys are: (1) powder metallurgy route based on rapid solidification and deformation consolidation; (2) equal channel angular extrusion/pressing (ECAE/ECAP). Though, by these techniques, magnesium alloys with submicron grains can be prepared, the production efficiency is low and the processing cost is high. Therefore, it is not feasible to use such techniques for large scale production.

Melt twin-rolling is a quite new technique for manufacturing of magnesium alloy sheets. By twin-rolling, magnesium alloy sheets with grains as fine as around 10 μm can be prepared, but the microstructure is still an as-cast microstructure in nature. In addition, for manufacturing thin sheets, subsequent hot or cold rolling processing is needed. What is more, both the sheet quality and the mechanical properties are very difficult to control, which often leads to unsatisfying products. Now it is commonly believed that melt twin-rolling is only suitable for manufacturing magnesium alloy plate billets, which are used for production of sheet products by further hot or cold rolling processes. In this way, the production cost can be reduced. However, whether this process can improve the plasticity of magnesium alloys is still unknown and needs further investigation.

Therefore, melt twin-rolling is still under investigation on laboratory scale in most countries such as USA, Germany, and China, despite its successful application to small scale production in Australia.

In summary, it can be said that before this invention there is no breakthrough in the techniques for manufacturing magnesium alloy sheets, and both the production cost and the market price of magnesium alloy sheets are too high for such products to find wide application.

SUMMARY OF THE INVENTION

The aim of the present invention is to develop a rolling technique for manufacturing magnesium alloy sheets with high plasticity which is featured by heating the rolls during rolling to ensure an unconventional decreasing temperature field from the surface to the center within the rolled billet material. It is expected that the invention can not only help to enhance the material utilization ratio, to increase the production efficiency, and to reduce the production cost, but also achieve significant improvement in plasticity of the rolled sheet products. This will make it possible for magnesium alloy sheets to be formed at room-temperature by secondary deformation processes.

The above-mentioned target can be realized by the following technical steps:

By heating the rolls during rolling, the said rolling process with an unconventional decreasing temperature field within the rolled billet can be realized, and the roll temperature can be in-situ monitored to make it match with the initial temperature field of the billet material.

The said unconventional decreasing temperature field rolling process means that before rolling the initial temperature in the surface of the billet material is higher than in its center, and during rolling there is heat transfer from the billet surface to its center.

During the said rolling process, the billet surface and central temperature is in the range of 250-450° C. and 20-150° C., respectively. The thickness reduction for each pass of rolling is 20-70% and the rolling speed is 5-10 m/min.

During the said rolling process, the billet surface temperature can be 450° C., 400° C., or 300° C. while the billet center is at room temperature.

During the said rolling process, the billet surface temperature can be 450° C. while the billet central temperature is 100° C.

For said the rolling process using as-cast magnesium alloys as starting billet materials, the thickness reduction for each pass of rolling is 20-30% for the first 2 passes, 30-40% from the 3^(rd) to the 6^(th) pass, 40-50% from the 7^(th) to the 10^(th) pass, and 50-60% for the further passes, respectively.

For the said rolling process using as-cast magnesium alloys as starting billet materials, the deformation amount for each pass of rolling is 20-30% for the first 2 passes, 30-40% from the 3^(rd) to the 5^(th) pass, and 30-60% for the further passes depending upon the requirement for the final sheet thickness.

BENEFITS OF THE INVENTION

1. For the conventional hot rolling of magnesium alloys, the starting and end rolling temperature recommended by handbooks for magnesium processing is 420-450° C. and 300° C., respectively. For each pass of hot rolling operation, the recommended deformation amount or the thickness reduction is 20% or so. If we choose an initial thickness of 120 mm for the starting as-cast billet, for the manufacturing of magnesium alloy sheets of 1-2 mm in thickness, then 26-28 rolling passes are needed, and the billet should be heated intermittently for at least 3-5 times. In contrast, the present invention introduces the unconventional decreasing temperature field rolling concept. By this new rolling process, for manufacturing magnesium alloy sheets with the same thickness as mentioned above, the billet material needs to be heated only once and the rolling operation are reduced to 13-14 passes because of the deformation ratio can be as high as 30-60% for each pass of rolling. Besides, the material utilization ratio can be enhanced correspondingly from 30-40% to 60-70%. What is more important, by the said unconventional decreasing temperature field rolling process, the grain size of the magnesium sheets can be reduced down to less than 10 μm, usually as fine as 2-6 μm in average size. Accordingly, the ductility of the sheets is significantly improved, with the tensile elongation achieving more than 21% (25-28% in most cases) in both longitudinal and transverse directions. In some circumstances, the tensile elongation can even be as high as 34%. In addition, the ductility difference in different directions can be controlled in the range of 10-15%.

2. The present invention has not only solved the problem of high production cost associated with the manufacturing of magnesium alloy sheets but also provided an effective way to improve their plasticity. On the one hand, the production cost for the said process is comparable with the conventional process for manufacturing of aluminum sheets. On the other hand, the plasticity of the magnesium sheets manufactured by the said process can achieve the level of Al—Mg alloy or mild low-carbon steel sheets. Therefore, it can be said that the present invention has solved the difficulties associated with the deformation processing of magnesium and magnesium alloys.

3. The present invention will eventually change the stereotyped thinking that magnesium and magnesium alloys can not be shaped at room-temperature by deformation processing because of their intrinsic poor plasticity. It opens up a new way to help widen and speed up the application of magnesium and magnesium alloys.

4. By the said unconventional decreasing temperature field rolling process disclosed in the present invention it is meant that during rolling the temperature in the surface of the billet material is higher than in its center. In such a case, the temperature field within the rolled billet is contrary to that encountered in the conventional hot or cold rolling process. That is to say, during the said unconventional decreasing temperature field rolling, heat flows from the billet surface to its center, while in the case of conventional rolling process the heat flow is in the opposite direction.

Even though magnesium and magnesium alloys are difficult to form by deformation processing due to their poor plasticity, the present invention managed to solve this problem by improving the plasticity and deformability of magnesium and magnesium alloys. For all conventional deformation forming processes, the billet usually has a temperature field with increasing temperature distribution toward its center, since there will be heat losses from the billet to the tools and the surroundings during deformation processing. In the present invention, on the contrary, the rolls are constantly heated during rolling to keep it at a temperature higher than that of the billet. Therefore, within the rolled billet there is an unconventional decreasing temperature field which is featured by a decreasing temperature from the billet surface to its center. In fact, based on the above concept, we can define the unconventional decreasing temperature field deformation processes as a new category of materials processing technology, and part of it is the said unconventional decreasing temperature field rolling method disclosed in the present invention.

The concept of unconventional decreasing temperature field deformation processing is proposed to overcome the drawbacks of the currently used conventional deformation forming processes. It is against the conventional hot or cold deformation processing practice in that heat transfers from the tools to the billet during deformation processing. Though the present invention was restricted to the rolling of magnesium and magnesium alloy sheets, the underlying theory related to the said unconventional decreasing temperature field rolling process is no doubt applicable for the deformation processing of other metallic materials. This means that the said unconventional decreasing temperature field rolling process disclosed in the present invention has a solid foundation in principle.

Technically, the present invention aims at the deformation processing of magnesium and magnesium alloys, in particular, the rolling of magnesium alloy sheets, because the rolling of magnesium and magnesium alloys is the most difficult.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION First Embodiment

For manufacturing high-plasticity magnesium and magnesium alloy sheets, the magnesium or magnesium alloy billet is rolled via the said unconventional decreasing temperature field rolling process, which is realized by heating the mill rolls during rolling to make the roll temperature match with the initial temperature field of the billet.

By the said unconventional temperature field is meant the initial temperature field of the billet upon rolling, wherein the billet surface has a higher temperature than its center and therefore the excessive heat transfers from the surface to the center of the billet.

Second Embodiment

In the application as claimed in example 1, the initial billet surface temperature is in the range of 250-450° C., while the central temperature is accordingly in the range of 20-150° C. The billet thickness reduction for each pass of rolling is 20-70%, and the rolling speed is 5-10 m/min.

Third Embodiment

In the application as claimed in example 1, the initial billet surface temperature is chosen to be 450° C., while the central temperature is 100° C.

Fourth Embodiment

In the application as claimed in examples 1, 2, and 3, with the use of cast magnesium ingot billets, the billet thickness reduction by each pass of rolling is 20-30% for the first 2 passes, 30-40% from the 3^(rd) to the 6^(th) pass, and 40-50% from the 7^(th) to the 10^(th) pass, and 50-60% for further passes, respectively.

Fifth Embodiment

In the application as claimed in examples 1, 2, and 3, with the use of cast magnesium ingot billets, the billet thickness reduction by each pass of rolling is 20-30% for the first 2 passes, 30-40% from the 3^(rd) to the 6^(th) pass, and 30-60% for further passes depending on the required final sheet thickness, respectively.

Sixth Embodiment

(1) Billet material: cast AZ31 Mg alloy plate ingot with a thickness of 100 mm is used as starting billet, and the width of the billet depends on the roll length of the mill. In the present example, the width and length of the billet is chosen as 400 mm and 600 mm respectively.

(2) Heating of the billet: the billet surface is heated to 400° C., but the billet center is kept at room-temperature (20° C.).

(3) Rolling speed: 5-10 m/min.

(4) Lubricating: plant oil or siliceous oil is sprayed on the surface of both the rolls and the billet.

(5) Deformation amount and rolling passes: the thickness reduction for each pass of rolling is controlled to be in the range of 30-60%, and 13-14 rolling passes are needed to obtain the final sheet product of 1 mm in thickness starting from the billet of 100 mm in thickness.

(6) Trimming of the edge, the starting and the end section of the sheet product.

(7) Material utilization ratio: 65-66%.

(8) Mechanical properties of the final sheet product:

In longitudinal direction: yield strength: 156 MPa; tensile strength: 261 MPa; elongation: 26%

In transverse direction: yield strength: 165 MPa; tensile strength: 255 MPa; elongation: 24%

Seventh Embodiment

(1) Billet material: cast ZK60 Mg alloy plate ingot, the dimension of which is 20 mm (thickness)×400 mm (width)×400 mm (length).

(2) Heating of the billet: the billet surface is heated to 400° C., but the billet center is kept at about 100° C.

(3) Rolling speed: 5-10 m/min.

(4) Lubricating: plant oil or siliceous oil is sprayed on the surface of both the rolls and the billet.

(5) Deformation amount and rolling passes: 8 rolling passes are needed to obtain the final sheet product of 0.4-0.5 mm in thickness with the use of billet of 20 mm in thickness. The deformation amount for different rolling passes is as follows:

For the 1^(st) pass, the thickness reduction is 20%;

For the 2^(nd) pass, the thickness reduction is 30%;

From the 3^(rd) to the 6^(th) pass, the thickness reduction is 40% for each rolling pass;

For the 7^(th) and the 8^(th) pass, the thickness reduction is 50% respectively;

(6) Trimming of the edge, the starting and the end section of the sheet product.

(7) Material utilization ratio: 64-66%.

(8) Mechanical properties of the final sheet product:

In longitudinal direction: yield strength: 230 MPa; tensile strength: 340 MPa; elongation: 34%

In transverse direction: yield strength: 250 MPa; tensile strength: 336 MPa; elongation: 28%

In examples 1 and 2, the control of the temperature field of the billet and the determination of the deformation amount for various rolling passes are the key technical issues.

The control of the unconventional decreasing temperature field can be achieved by controlling the initial surface and center temperature of the billet. Upon rolling, the temperature field within the billet will change, and the temperature gradient will even disappear due to both the heat transfer between the billet and the rolls and the heat released by plastic deformation. The technical parameters which were given in the claimed application examples are references to special cases. In practice, both the initial temperature field of the billet and the deformation amount for any rolling pass should be adjusted referring to the alloy composition, the billet dimension, and the final sheet thickness, etc.

The determination of the deformation amount for any pass of rolling should be based on the initial microstructure of the cast billet and the alloy composition. Usually, for the first rolling pass, the deformation amount or thickness reduction should be relatively smaller than that for the subsequent rolling passes. The deformation amount for each pass of rolling is suggested to be in the range of 20-70%. 

1. A rolling technique for manufacturing magnesium alloy sheets with high plasticity which is characterized in that by heating rolls during rolling to ensure an unconventional decreasing temperature field from the surface to the center within a rolled material. The said heating makes it a reality for a roll temperature to match with an initial temperature field within a billet material to be rolled.
 2. A rolling technique as claimed in claim 1, wherein the said unconventional decreasing temperature field means that before rolling the initial temperature in the surface of the billet material is higher than in its center, and thus upon rolling there is heat transfer from the billet surface to its center.
 3. A rolling technique as claimed in claims 1, wherein the billet surface and central temperature upon rolling is in a range of 250-450° C. and 20-150° C., respectively. A thickness reduction for each pass of rolling is between 20-70% and a rolling speed is between 5-10 m/min.
 4. A rolling technique as claimed in claims 2, wherein the billet surface and central temperature upon rolling is in a range of 250-450° C. and 20-150° C., respectively. A thickness reduction for each pass of rolling is between 20-70% and a rolling speed is between 5-10 m/min.
 5. A rolling technique as claimed in claims 1, wherein the said billet surface temperature during rolling is selected from 450° C., 400° C., or 300° C. while the billet center is still at room temperature.
 6. A rolling technique as claimed in claims 2, wherein the said billet surface temperature during rolling is selected from 450° C., 400° C., or 300° C. while the billet center is still at room temperature.
 7. A rolling technique as claimed in claims 1, wherein the said billet surface temperature during rolling is 450° C. while the billet central temperature is 100° C.
 8. A rolling technique as claimed in claims 2, wherein the said billet surface temperature during rolling is 450° C. while the billet central temperature is 100° C.
 9. A rolling technique as claimed in claims 3, wherein for the rolling processes with as-cast magnesium alloys as starting billet materials, a thickness reduction for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 6^(th) pass, 40-50% from a 7^(th) to a 10^(th) pass, and 50-60% for further passes, respectively.
 10. A rolling technique as claimed in claims 4, wherein for the rolling processes with as-cast magnesium alloys as starting billet materials, a thickness reduction for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 6^(th) pass, 40-50% from a 7^(th) to a 10^(th) pass, and 50-60% for further passes, respectively.
 11. A rolling technique as claimed in claims 5, wherein for the rolling processes with as-cast magnesium alloys as starting billet materials, a thickness reduction for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 6^(th) pass, 40-50% from a 7^(th) to a 10^(th) pass, and 50-60% for further passes, respectively.
 12. A rolling technique as claimed in claims 3, wherein for rolling of as-cast magnesium alloys billets, a deformation amount for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 5^(th) pass, and 30-60% for further passes depending upon a requirement for a final sheet thickness.
 13. A rolling technique as claimed in claims 4, wherein for rolling of as-cast magnesium alloys billets, a deformation amount for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 5^(th) pass, and 30-60% for further passes depending upon a requirement for a final sheet thickness.
 14. A rolling technique as claimed in claims 5, wherein for rolling of as-cast magnesium alloys billets, a deformation amount for each pass of rolling is 20-30% for a first 2 passes, 30-40% from a 3^(rd) to a 5^(th) pass, and 30-60% for further passes depending upon a requirement for a final sheet thickness. 